Production of dislocation-free silicon single crystals



P. H. HUNT Aug. 13, 1968 PRODUCTION OF DISLOCATION-FREE SILICON SINGLE CRYSTALS Filed Aug. 27, 1965 W/QM @fffwmey United States Patent 3,397,042 PRODUCTION OF DlSLOCATION-FREE SILICON SINGLE CRYSTALS Patrick H. Hunt, Richardson, Tex., assignor to Texas Instruments Incorporated, Dallas, Tex., a corporation of Delaware Original application Oct. 15, 1963, Ser. No. 316,347, now Patent No. 3,275,417, dated Sept. 27, 1966. Divided and this application Aug. 27, 1965, Ser. No. 493,955

4 Claims. (Cl. 23301) ABSTRACT OF THE DISCLOSURE Disclosed are apparatus and methods for the production of dislocation-free crystals of semiconductor material. A semiconductor rod is supported at the bottom thereof in a controlled atmosphere within a nonconductive tube. Means including a single turn, inductive heating member encircles the rod outside the tube and is mounted for relative movement with respect to the rod for production of a molten zone in the rod when energized by a frequency alternating current. A cooled, single turn, shorted focusing coil encircles the axis of the rod immediately below the heating member to control the molten zone from limiting the character of the height thereof. After a molten zone is created in the top part of the rod, a single crystal seed is dipped into the molten zone and the seed is rotated while withdrawing the same from the molten zone at a withdrawal rate initially substantially in excess of the upward movement of the rod and then at a rate substantially corresponding with the rate of movement of the rod.

This application is a division of copending application, Ser. No. 316,347, filed Oct. 15, 1963, now Patent No. 3,275,417, dated Sept. 27, 1966.

This invention relates to a float zone process for forming silicon single crystals and more particularly to the control of a float zone crystal growth in which control is maintained over a molten Zone and a freezing solidliquid interface to minimize crystal dislocations.

In the manufacture of silicon semiconductor devices, high quality single crystals of silicon are necessary as starting materials. In accordance with one known method of growing crystals, a silicon seed is dipped into a molten pOOlOf silicon contained in a quartz crucible and then slowly withdrawn. It has been found that resulting crystals contain approximately atoms per cc. of oxygen which is derived from reaction of the molten silicon with the quartz crucible. This leads to undesirable heat treating effects as to the resistivity of the silicon body during subsequent processing. Thus, it is desirable to eliminate the high oxygen content.

In float zone processes of the type generally disclosed in Keller Patent No. 2,992,311, a molten zone is caused to move along the length of an elongated silicon rod which is held in a vertical position. This method has the advantage of producing low oxygen content crystals, but has been found to be disadvantageous in that any crystal of reasonable diameter, such as above about one centimeter, contains a large number of dislocations. The dislocations lower the minority carrier lifetime and further act as preferential diffusion paths for impurities during further processing and for the foregoing reasons are highly undesirable. In Dash Patent No. 2,961,305, contact between a molten zone and a single silicon crystal is established at the top pedestal of silicon which is longitudinally slotted. This slotting process and the fact that only small diameter crystals can be grown from the pedestal makes the system somewhat limited as a production operation although structurally perfect crystals may be obtained.

In accordance with one aspect of the present invention,

3,397,042 Patented Aug. 13, 1968 there is provided a system for transfer to the form of a dislocation-free crystal of semiconductor material from a rod supported at the bottom thereof in a controlled atmosphere within a non-conductive tube onto a seed crystal supported for rotation above and at the axis of the rod. Means including a single-turn, inductive heating member encircles the rod outside the tube and is mounted for relative movement with respect to the rod for production of a molten zone in the rod when energized by high frequency alternating current. A cooled, single-turn, shorted focusing coil encircles the axis of the rod immediately below the heating member to control the molten zone for limiting the character and the height thereof.

In a more specific aspect, the focusing coil is in the form of an upwardly pointing truncated cone having a central aperture through which the rod passes.

. In accordance with a further aspect of the invention, there is provided a method for crucible-free float zone forming of a dislocation-free crystal from a rod of semiconductor material. The rod is supported from the bottom in substantially vertical orientation in an inductively coupled R.F. field to form a molten zone in the rod which is sharply limited on the lower boundary thereof. A single crystal silicon seed is dipped into the molten zone and then, while under rotation, the seed is withdrawn from the zone at a rate initially substantially in excess of the movement of the rod upward through the field and then at a rate substantially corresponding with the rate of movement of the rod through the field.

For a more complete understanding of the present invention and for further objects and advantages thereof, reference may now be had to the following description taken in conjunction with the accompanying drawings in which:

FIGURE 1 is a diagrammatic View of an apparatus embodying the present invention;

FIGURE 2 is a perspective view of the coil 13 and its support; and

FIGURE 3 is an enlarged view of a seed and molten zone shortly after initiation of crystal growth.

FIGURE 1 illustrates a system for float zone crystal growth in accordance with the present invention. A rotating single crystal seed 10 of silicon initially is dipped into a molten pool formed on the top of a rod 11 of silicon. Radio frequency power coupled to the silicon rod by a single-turn, closely-coupled coil 13 is adjusted until the tip of the seed melts, forming a continuous molten zone between the rod 11 and the seed 10. The seed 10 is then withdrawn from the melt, and the R.F. power and the withdrawal speed are adjusted so that a long neck 14 is formed below the seed 10. The neck 14 is of diameter smaller than that of the seed. After the neck 14 is of length of at least about one to two centimeters, the R.F. power and the seed withdrawal rate are then changed gradually to preset values, during which time a transition zone 15 is formed. Predetermined values of withdrawal and feed are reached which determine the diameter of the newlyformed crystal 17 above the melt zone 16.

In accordance with the present invention, control is maintained over the heating zone and partially over the geometry of the RF field sharply to limit the lower boundary of the field. This permits control over the molten zone thereby to assure a minimum occurrence of dislocations.

The system for carrying out crystal growth in accordance with the preferred embodiment of the invention is shown in FIGURE 1. In this embodiment, the lower end of rod 11 is supported in a chuck, which chuck is supported by a bottom mounting ring 20. Ring 20 is supported at the lower end of a rectangular frame 21 having two upper crossbars 22 and 23. The frame 21 is supported at opposite sides thereof on lead screws such as the screw and by followers 26 and 27. The lead screw 25 is supported at its lower end by a member 30 of a fixed frame or support. A second lead screw (not shown) is similarly supported at its lower end. A motor 31 is illustrated as con nected to lead screw 25 by way of bevel gears 32. A similar driving arrangement is provided for the other lead screw. Preferably, the motor 31 will drive both lead screws so that the frame 21 may be precisely controlled in its movement upward relative to coil 13.

The seed 10, of rectangular cross section, is mounted in a chuck 35 and is solely supported in chuck 35 as to be movable relative to the frame 21. The chuck 35 is supported at the lower end of a shaft 36. The shaft 36 is journaled in an upper housing member 37 and is provided with a drive pulley 38. The shaft 36 may be rotated by way of belt 39 driven from a motor 40. The motor 40 is mounted on a cross plate 41 which extends between a pair of followers 42 and 43. Followers 42 and 43 are mounted on lead screws 44 and 45, respectively. The lower ends of the lead screws 44 and 45 are journaled in the cross bars 22 and 23. A pulley 46 is mounted .on the lead screw member 44 between crossbars 22 and 23 and is coupled by way of a belt 47 to a first pulley on the shaft of a motor 48. In a similar manner, a second pulley 49 mounted on the lower end of the lead screw 45 is coupled by way of a belt 50 to a second pulley on the shaft of the motor 48. Pulleys 46 and 49 are keyed to the lower ends of the lead screws 44 and 45, respectively, for transmission of driving forces thereto. As the motor 40 is energized, the shaft 36 carrying the chuck 35 is caused to rotate in order to rotate the seed 10. As the motor 48 is energized, the lead screws 44 and 45 are rotated, causing the followers 42 and 43 to be raised or lowered, depending upon the direction of rotation of the motor 48. Thus, the seed 10 may be raised or lowered relative to the upper end .of the rod 11.

The housing 37 in which the shaft 36 is journaled is coupled by way of a flexible bellows 51 to the upper end of a mounting ring 52. The shaft 36 may be slidably supported in a bearing in the mounting ring 52. An auxiliary hand-powered drive arrangement is provided for raising and lowering the seed 10. A crank wheel 53 is coupled by bevel gears 54 to the shaft .of motor 48 so that an adjustment can be made as to the elevation of the seed 10 without energizing motor 48.

A quartz tube is clamped in the rings 20 and 52 and is coaxial with the rod 11. Thus, all of the structure carried by frame 21 is movable relative to coil 13 so that a molten zone initiated in rod 11 may be caused to travel along the length of the rod 11 by raising the frame 21. Preferably, the travel of the frame 21 is carefully controlled as to be of constant speed so that mechanically induced aberrations may be avoided.

In accordance with the present invention, the character of the freezing interface between the melt zone 16 and the newly-formed crystal 17 and the starting rod 11 is maintained horizontal and flat by control of the molten zone. This is accomplished, in accordance with the present invention, by control of the heating pattern from the coil 13 and by the movement of the newly-formed crystal 17 relative to the rod 11. More particularly, the coil 13 is a hollow, single-turn copper loop such as shown in FIGURE 2. It is mounted on a pair of brackets and 71 which are insulated one from the other. Coil 13 is flow-connected at the ends of the single turn to a source of cooling water, the flow of which is maintained through the coil. Coil 13 is also connected to a source of RF. power at frequency of about 4 megacycles and capacity of about 25 kilowatts. In addition to this, focusing coil 72 is mounted immediately below the coil 13 and is in the form of a hollow, truncated cone having a center bore through which the tube 60 extends. The focusing coil 72 is made of a highly conductive material such as copper, and is provided with a pair of flow channels 73 and 74 leading thereto. Flow channels 73 and 74 provide for flow of cooling water through the focusing coil 72. The coil is placed immediately adjacent to and immediately below the coil 13 and serves to rob power from the field produced by excitation of the coil 13. This serves to control the length of the molten zone and limits its shape. The control .over the configuration of the freezing zone above the coil 13 has been found to be controllable by this means so that the crystal 17 may be grown above rod 11 of diameter equal the diameter .of rod 11 without the molten zone 16 falling out and with substantial freedom of dislocations.

A continuous flow of an inert gas, such as argon, is maintained through the tube 60 during the crystal growing cycle. Argon is introduced into the lower end of the tube 60 by way of a channel 75. Argon flows from the upper end of the system by way of channel 76.

The growing cycle is initiated by playing a flame from a hydrogen torch 77 onto the wall of the tube 60 in a region preferably somewhat below the upper end of the rod 11. As the rod 11 is heated, the coil 13 is energized from a radio-frequency generator so that coil 13 becomes electromagnetically coupled to the rod 11. The coupling is such that the rod may be made molten by increasing the energy level of the field from coil 13. The flame from torch 77 is then turned off. The rod 11 is then lowered so that the R.F. coupling zone is at the top .of rod 11. The RP. power is then increased.

As the rod 11 becomes molten, the seed 10 is lowered so that the lower end thereof is immersed in the molten pool on the upper end of the rod 11. Surface tension of the molten zone 16 and the electromagnetic field from the coil 13 maintains the integrity of the molten zone preventing its falling out. The seed 10 is then rotated at a relatively slow speed and is slowly withdrawn from the upper surface of the molten pool on rod 11. The motors 31 and 48 are adjusted as to speed so that the crystal initially grown onto the seed 10 is necked down to a smaller diameter than the seed 10 and is permitted to maintain the relatively small diameter until it attains a length of several centimeters. At this time, the rate of movement of the seed 10 relative to the rod 11 is reduced gradually so that the transition zone 15 is formed. In operation, it has been found that the movement of the seed 10 relative to the rod 11 may be discontinued so that the newly-formed section 17 will have the same diameter as the original rod 11. The motor 31 then continues to drive the frame 21 upward, carrying with it the rod 11 as encased Within the quartz tube 60. Thus, the molten zone 16 moves downward through the entire length of the rod 11 with the newly-formed crystal 17 freezing progressively above the molten zone as the body moves away from the zone controlled by the coil 13. A single pass beginning at the top .of the rod 11 and terminating at the bottom of the crystal has been found to produce a crystal 17 which is free from dislocations and has a relatively low oxygen content and high minority carrier lifetime.

The traverse of the molten zone 16 down the length of the rod 11 may be of the order of about one hundred and fifty millimeters per hour with no differential pull rate and with the rotation of the seed 10 and the newly-formed crystal 17 in the range of from about two (2) to twenty (20) rpm. The rotational speed preferably is in the lower end of the latter range. Crystals may be grown having diameters .of two (2) to three (3) centimeters or more while maintaining the freedom from dislocations. The flow rate of gas by way of channel 75, 60, and 76, at a rate of about one-half liter per minute, has been found to be satisfactory. Higher flow rates are employed when the diameter .of the newly-formed crystal section 17 reaches a diameter of about 3.3 millimeters. In the latter range, flow rates of one to 1.5 liters per minute will be employed.

In a typical example, the rod 11 may have a diameter of about twenty-eight (28) millimeters. The quartz tube 60 would have an inner diameter of thirty-two (32) millimeters and an outer diameter of thirty-four (34) millimeters. The coil 13 is about five (5) millimeters tube size and of diameter such that the quartz tube 60 is free of the coil 13 but with minimum spacing therebetween. The focusing coil 72 is of the same inner diameter as the inner diameter of the coil 13. The upper surface of the coil 72 was spaced about ten millimeters below the lower surface of the coil 13. Coil 72 was of the order of one hundred and twenty-seven (127) millimeters in diameter at the bottom and slightly larger than the coil 13 at the top thereof.

In the foregoing example, the rod 11 was about twentyeight (28) millimeters in diameter. The travel of the rod through the coil 13 was maintained at about one hundred and fifty (150) millimeters per hour. In instances where larger diameter crystals have been employed, such as thirty-three (33) millimeter crystals, the speed has been lowered somewhat to about one hundred and fifteen (115) millimeters per hour in order to assure a zone which is completely molten, and to avoid freezing up during the pulling operation. The traverse rate will thus depend upon the size of the rod employed and upon the power in the RF. field. It should therefore be understood that the foregoing is given by way of example and not by way of limitation.

In order to form dislocation-free crystals, it is necessary that the seed 10 be thoroughly wetted in the molten zone on top of the rod 11 before beginning to pull the crystal. A guide by which the operation may be evaluated is illustrated in FIGURE 3. The seed 10 is withdrawn from the molten top of rod 11 to form the neck 14 of reduced diameter to assure crystal structure of a large diameter which is dislocation-free. Crystal growth prefer ably is initiated when the lower end of seed 10 is of the shape illustrated in FIGURE 3. More particularly, as the seed 10 rotates and becomes wetted in the melt, the corners of the seed tend to form cusps with downwardly pointed corners. When the seed 10 is shaped generally in the form indicated in FIGURE 3, the pulling operation may then be commenced.

While not shown in the drawings, the system includes electrical controls for the speeds of motors 31, 40, and 48 as well as for control of the level of power from a suitable R.F. generator for the coil 13 of FIGURES 1 and 2. Such controls are well-known in the art.

It is to be understood that the system operate to transfer semiconductor material from the bottom supported rod 11 into the form of a dislocation-free crystal. The operation is carried out in the argon atmosphere in the tube 60. The transfer, by way of a molten zone of controlled configuration, is onto a single crystal seed rotatably supported above the molten zone. The singleturn annular heating coil 13 encircles the rod 11 outside the tube 60' and is energized by high frequency alternating current to create the molten zone in the rod 11. The single-turn, shorted focusing coil 72 encircles rod 11 immediately below the heating coil 13 to rob power from the RP. field at the lower boundary to limit the height of the molten zone and thus control the molten zone.

The foregoing description has related primarily to the growth of crystal structures of silicon base materials. It will be recognized that other semiconductor materials may be similarly treated to produce dislocation-free structures. In the growth of germanium crystals, the primary problem attendant to the production of dislocation-free silicon crystals is not present. Thus, germanium can be grown from melts contained in quartz crucibles. Nevertheless, the present invention may be employed for production of germanium bodies which are dislocation-free, if desired. Since the density of germanium is somewhat greater than that of silicon, the diameter of the float zone would have to be maintained somewhat smaller in order to maintain its integrity. In providing an inert atmosphere in which the molten zone is maintained, argon has been used and has been specified in the foregoing description. However, any inert gas which will not react with silicon at the temperatures involved would be suitable. Helium, neon, and the like, as well as hydrogen,

are satisfactory.

Having described the invention in connection with certain specific embodiments thereof, it is to be understood 5 that further modifications may now suggest themselves to those skilled in the art and it is intended to cover such modifications as fall within the scope of the appended claims.

What is claimed is:

1. A method of growing a dislocation-free single crystal solid rod by crucible-free float zone melting in an inert atmosphere of a rod of semiconductor material which comprises:

(a) enclosing said rod in a quartz housing,

(b) supporting said rod from the bottom in substantially vertical orientation,

(c) establishing an inductively coupled field through said housing to form a molten zone in said rod which is sharply limited on the lower boundary thereof,

(d) dipping a single crystal silicon seed into said molten zone,

(e) rotating said seed while withdrawing the same from said molten zone, at a withdrawal rate initially substantially in excess of the movement of said rod upwardly through the field and then at a rate substantially corresponding with the rate of movement of said rod through said field,

(f) flowing inert gas through said housing, and

(g) transporting heat from said rod and from said molten zone at the lower boundary of said field by continuous liquid coolant flow through said field.

2. The method of growing a dislocation free single crystal solid rod by zone melting a semiconductor rod in which the rod is vertically supported at its lower end in a controlled atmosphere which comprises:

' (a) applying a heating and levitation field to a limited zone at a first point along said rod,

(b) moving said rod relative to said field to position the top of said rod in said field to form a molten zone at the top of said rod,

(c) dipping a single crystal seed of silicon into said zone to melt the lower end thereof,

((1) withdrawing said seed while moving said seed and said rod differentially upward with respect to said field to form a small neck below said seed,

(e) rotating said seed while withdrawing the same from said molten zone, at a withdrawal rate initially substantially in excess of the movement of said rod upwardly through the field and then at a rate substantially corresponding with the rate of movement of said rod through said field,

(f) reducing the differential rate as the molten zone moves downward along said rod to produce a crystal above said molten zone of diameter determined by said differential rate, and

(g) transporting heat from said rod and from said molten zone at the lower boundary of said field by continuous liquid coolant flow through said field.

3. The method of forming a dislocation-free, single 60 crystal solid rod of semiconductor material from a vertical bottom-supported cylindrical semiconductor rod in a nonconductive tube which comprises:

(a) flowing inert gas through said tube,

(b) heating a limited zone along the length of said 65 rod to render the same conductive,

(c) subjecting said limited zone to a radio frequency electromagnetic field having a sharp lower boundary to maintain said zone conductive by flow of induced currents in said rod,

((1) moving said rod downward relative to said field to position the top of said rod in said field,

(e) increasing the power of said field to cause the top of said rod to form a molten zone of said semiconductor material,

(f) inserting a monocrystalline seed of semiconductor material of polyhedron cross section into said molten zone to form a wetting zone of very small crosssectional area as compared to the cross-sectional ar a of said rod,

(g) withdrawing said seed, after the development of downwardly extending peaks on the corners thereof, at a rate substantially in excess of the rate of feeding said rod upward through said field to form an elongated neck of solid, semiconductor material above said molten zone, said seed being rotated as material of rectangular cross section into said molten zone to form a wetting zone of very small crosssectional area as compared to the cross-sectional area of said rod,

(g) withdrawing said seed, after the development of downwardly extending peaks on the corners thereof, at a rate substantially in excess of the rate of feeding said rod upward through said field to form an elongated neck of solid, semiconductor material above said molten zone, said rod being rotated as it is withdrawn, 10 it is withdrawn,

(h) gradually reducing to zero the differential between (h) gradually reducing to zero the differential between the withdrawal and fed rates of said seed and said the withdrawal and feed rates of said seed and said rod, respectively, for forming a transition in said rod, respectively, for forming a transition in said crystal above said molten zone from a small diameter crystal above said molten zone from a small diameter to a diameter substantially equal to the diameter of to a diameter substantially equal to the diameter said rod, and of said rod, and

(i) moving said rod upward through said field at a (i) moving said rod upward through said field at a uniform rate to transfer said semiconductor mateuniform rate to transfer said semiconductor material rial from said rod to said crystal by way of said from Said rod to Said Crystal y y Of Said molten molten zone, zone,

(j) transporting heat from said rod and from said (1') ransporting heat from said rod and from said molten zone at the lower boundary of said field by molten zone at the lower boundary of said field by continuous liquid coolant flow through said field. continuous liquid coolant flow through said field.

4. The method of forming a dislocation-free, single crystal solid rod of semiconductor material from a bot- References Cited tom-supported cylindrical semiconductor rod supported UNIT STATES PATENTS vertically in a non-conductive tube which comprises:

(a) flowing inert gas through said tube, i 23 273 eller 2330l (b) heating a llmited zone along the length of said 2962 363 11/1960 M 23 30 rod to render the same conductive, 2981687 4/1961 P m 1 aimee 23--301 (c) sub ecting said limited zone to a radio frequency 3 135 585 6/1964 Dash 23 273 electromagnetic field having a sharp lower boundary 3157472 1/1964 K I thereof to maintain said zone conductive by flow of appe meyer 2 273 induced current in said rod, FOREIGN PATENTS (d) moving said rod downward relative to said field 674121 11/1963 Canada.

i0 'POSitiOII the top Of said rod in said field, 1 930:432 7 19 3 Great Britain (c) increasing the power of said field to cause the top of said rod to form a molten zone of said semiconductor material,

(f) inserting a monocrystalline seed of semiconductor NORMAN YUDKOFF, Primary Examiner.

G. HINES, Assistant Examiner. 

